Contributed by ​Robin (Banner) McLachlan, ​​PhD student at University of Washington

Corals are being stressed by so many things nowadays. For example, they must cope with living in warmer water that can lead to coral bleaching and die-off. On top of this, humans are making the situation even worse by dredging up coral reefs and smothering nearby reefs with the dredged sediment.

Sedimentation on reefs can harm coral communities by reducing coral recruitment, survival, and the settlement of new coral larvae. Corals near the Port of Miami were hit simultaneously with both a warming event and sediment dredging a few years ago, and scientists are now finding out that the sediment dredging practice is harming the corals because dredging has not been well managed nor monitored.

A federal channel at the Port of Miami was dredged between 2013 and 2015 in order to make shipping lanes wider and deeper, a necessary effort for safer navigation. But dredging has both good and bad effects. Over 3 acres of coral reef were ground up and surrounding reef communities were blanketed by the dredged sediment. The Army Corps of Engineers, the agency that held the dredging permit, admitted that there would be harmful impacts from dredged sediment settling onto nearby reefs. However, they didn’t attempt to quantify the impacts, and the project only required the health of corals to be monitored within 50 meters of the dredged channel.

After the dredging project, scientists funded by the National Oceanic and Atmospheric Administration (NOAA) attempted to quantify the effects of the project by comparing corals that had been impacted by both the warming event and sedimentation with those that had only suffered through the warming event. They did this by diving on coral communities to perform posthoc surveys of sediment thickness and coral health as well as comparing images of corals pre-, during-, and after- dredging.

These scientists found that sedimentation occurred up to 700 m from the dredged channel, 14 times farther away than original monitoring extended. Additionally, they found that corals that were covered with sediment were 4 times more likely to lose tissue and 3 times more likely to die than corals that were not covered.

​This coral die-off was extreme because the coral reef communities were hit with two stressors at the same time: warm temperatures and sedimentation. Although it may not be possible to stop these warm water events, it is possible to time dredging projects so that they don’t happen at the same time. The scientists that performed this study argue that this coral die-off was exacerbated by the dredging project and urge that future projects should be scheduled so not to overlap with temperature stress.

With a new project already planned to expand the port at Port Everglades, which is less than 40 km north of Port of Miami, we will soon see how their pleas are answered.